Rare and noble metal resources in China are extremely scarce, comprising only 0.48% of global reserves. Consequently, the technology for comprehensive utilization of rare and noble metals is one of the key technologies that support the development of high-tech industries in China such as national defense and military, aeronautics and astronautics, new energy sources, information, environmental protection and the like.
Rare and noble elements, which have low reserves and are extremely scattered under natural conditions, are not worth being extracted directly from raw ores. However, since scattered rare and noble metals are mostly associated with minerals of nonferrous metals such as copper, lead, zinc and the like, their deep and comprehensive recovery from byproducts, especially anode slime, of nonferrous metal industry is an important approach to the development of rare and noble metals at home and abroad.
Anode slime is the sediment produced at the bottom of an electrolytic tank during industrial electrolysis and an electrolytic byproduct comprising noble metals and recoverable valuable metals.
With the components in anode slime fluctuating considerably, in general the total content of main elements such as gold, silver, lead, copper, antimony, arsenic, bismuth and so on reaches 70% or higher. Which process is ultimately employed for treating the anode slime is selected depending on specific circumstances such as the production scale, the site and the capacity turnover condition of an enterprise, as well as the components in the anode slime. Currently, processes for treating copper anode slime mainly comprise: (1) pyrometallurgical process; (2) hydrometallurgical process; and (3) pyro-hydrometallurgical combined process.
The specific process procedure of hydrometallurgical process comprises: selenium distillation by sulfatizing roasting, —copper extraction with a diluted acid—gold extraction by chlorination—silver extraction with sodium sulfite—gold and silver electrolysis. The specific process procedure of pyrometallurgical process comprises: pressure leaching of copper and tellurium—pyrometallurgical smelting—pyrometallurgical converting—silver electrolysis—gold extraction through treating silver anode slime. Currently, a relatively advanced process for treating anode slime in the world is the process in ronnskar smelter, Sweden, wherein the overall yield of finished gold and silver from anode slime are 98.5% and 98.5%, respectively. Most smelters in China use pyrometallurgical process or pyro-hydrometallurgical combined process.
Regardless of being domestic or overseas, currently, both pyrometallurgical process and pyro-hydrometallurgical combined process for treating anode slime comprise enriching gold and silver in anode slime by using lead, and smelting a noble lead after the noble lead is formed; meanwhile, reduction smelting and oxidation converting are mostly carried out in two metallurgical furnaces, leading to a long smelting time, a large amount of returned material and serious overstock of gold and silver. Meanwhile, the high toxicity of both lead and lead compounds causes great hazards to the environment and operators.